1. Technical Field
The present invention relates to a dental filler comprising microparticles of amorphous inorganic oxide prepared by covering surfaces of silica-based fine particles with a composite metal oxide, a method for producing such dental filler, and also a dental composite material containing the dental filler and a hardenable resin.
2. Background Technology
Generally, raw materials for preparing dental fillers are required to have a sufficient strength or hardness equivalent to those of natural teeth, a surface smoothness and a sufficient resistance against abrasion as caused by teething, and therefore silica-based fine particles have been used as a raw material for the dental fillers. Furthermore, the dental fillers are required to have compatibility in color tone with natural teeth, conformity in a refractive index for providing transparency equivalent to that of natural teeth, and an X-ray radiopacity enabling differentiation of a portion treated or repaired with the dental materials from a tooth tissue of natural teeth, and therefore zirconium oxide is also used as a raw material for the dental fillers.
Various examples of the dental fillers are disclosed in publicized documents, for instance, (1) the dental filler which is prepared by aggregating silicon dioxide and other metal oxide (such as zirconium oxide) and then subjecting the aggregated ones to a thermal treatment at a temperature not causing crystallization of the metal oxides, to obtain the aggregated particles of amorphous inorganic oxides such as silicon dioxide and other metal oxide (Refer to Patent document 1), and (2) the dental filler containing a substantially amorphous cluster constituted by fine particles of non-heavy metal oxide (such as silica particles) having the average diameter of less than about 100 nm and fine particles of heavy metal oxide (such as zirconium oxide particles) having the average diameter of less than about 100 nm (Refer to Patent document 2).
However, because the above dental fillers are prepared by mixing a silica sol (as a non-heavy metal oxide) and an aqueous solution of zirconium salt (as a source of a heavy metal oxide), drying the mixture solution with use of a spray drier or the like and then calcining the dried materials, they are obtained as the aggregated particles of amorphous einorganic oxides such as silicon dioxide and zirconium oxide having different refractive indexes. As a result, the refractive indexes measured in samples of the aggregated particles are not equal and inconsistent. Therefore, it is difficult to improve the transparency at the treated or repaired portion of a tooth (including an artificial tooth). Furthermore, the pore volumes of the aggregated particles and also the strength thereof are not adjustable to a desired value. Moreover, because the adhesiveness of the aggregated particles to hardenable resins is insufficient, the strength and the hardness at the treated or repaired portion of a tooth (including an artificial tooth) are lowered, which may lead to insufficient resistance of the dental fillers against abrasion as caused by teething.
To solve the problems as described above, the present inventor developed a method for producing a dental filler by mixing a silica sol, an aqueous solution of silicic acid and an aqueous solution of a zirconium salt, drying the mixture solution with a spray drier, and then calcining the dried mixture, and they filed a patent application for this method (Refer to Patent document 3). With this invention, it is possible to obtain a dental filler in which the silica component originated from the silicic acid and the acidic zirconium component are well mixed with each other, and therefore properties of the aggregated particles obtained (such as the refractive index) are close to those of silica particles originated from the silica sol. However, even in this dental filler, as the silica particles originated from the silica sol are still mixed therein, the problems as described above have not completely been solved yet.
Furthermore, for solving the problems relating to the conventional type of dental fillers as described above, the present inventors developed a dental filler containing microparticles of a crystalline inorganic oxide comprising a zirconium silicate compound, and they filed a patent application for this dental filler (Refer to Patent document 4). This dental filler has excellent properties such as the strength or hardness substantially equivalent to those of natural teeth, or high resistance against abrasion as caused by teething. However, when the dental filler is used in some specific applications, the microparticles of the crystalline inorganic oxide are required to subject to a surface treatment with an organic metal oxide such as an organic silicon compound, an organic titanium compound, an organic zirconium compound or the like. However, because the microparticles of the crystalline inorganic oxide have a surface property for crystalline particles, it is not always easy to homogeneously treat the surface with the organic metal compound.
Under the circumstances, the present inventors have made strenuous research and development efforts for solving the problems as described above, and found that microparticles of amorphous inorganic oxide prepared by covering surfaces of silica-based fine particles with a composite metal oxide have the excellent properties as a dental filler, and they completed the present invention.
Patent document 1: JP H07-196428
Patent document 2: JP 2003-512406 (WO 01/030306)
Patent document 3: JP 2003-146822
Patent document 4: Japanese Patent Application No. 2006-086800